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From: Andy Lutomirski <luto@kernel.org>
To: X86 ML <x86@kernel.org>
Cc: Borislav Petkov <bpetkov@suse.de>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Brian Gerst <brgerst@gmail.com>,
	Dave Hansen <dave.hansen@intel.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Josh Poimboeuf <jpoimboe@redhat.com>,
	Andy Lutomirski <luto@kernel.org>
Subject: [PATCH v3 13/19] x86/asm/64: Return to userspace from the trampoline stack
Date: Thu, 23 Nov 2017 20:32:59 -0800
Message-ID: <d350017000eed20922c3b2711a2d9229dc809256.1511497875.git.luto@kernel.org> (raw)
In-Reply-To: <cover.1511497874.git.luto@kernel.org>
In-Reply-To: <cover.1511497874.git.luto@kernel.org>

By itself, this is useless.  It gives us the ability to run some final
code before exit that cannnot run on the kernel stack.  This could
include a CR3 switch a la KAISER or some kernel stack erasing, for
example.  (Or even weird things like *changing* which kernel stack
gets used as an ASLR-strengthening mechanism.)

The SYSRET32 path is not covered yet.  It could be in the future or
we could just ignore it and force the slow path if needed.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
---
 arch/x86/entry/entry_64.S | 55 +++++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 51 insertions(+), 4 deletions(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 7d47199f405f..426b8c669d6a 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -330,8 +330,24 @@ syscall_return_via_sysret:
 	popq	%rsi	/* skip rcx */
 	popq	%rdx
 	popq	%rsi
+
+	/*
+	 * Now all regs are restored except RSP and RDI.
+	 * Save old stack pointer and switch to trampoline stack.
+	 */
+	movq	%rsp, %rdi
+	movq	PER_CPU_VAR(cpu_tss + TSS_sp0), %rsp
+
+	pushq	RSP-RDI(%rdi)	/* RSP */
+	pushq	(%rdi)		/* RDI */
+
+	/*
+	 * We are on the trampoline stack.  All regs except RDI are live.
+	 * We can do future final exit work right here.
+	 */
+
 	popq	%rdi
-	movq	RSP-ORIG_RAX(%rsp), %rsp
+	popq	%rsp
 	USERGS_SYSRET64
 END(entry_SYSCALL_64)
 
@@ -633,10 +649,41 @@ GLOBAL(swapgs_restore_regs_and_return_to_usermode)
 	ud2
 1:
 #endif
-	SWAPGS
 	POP_EXTRA_REGS
-	POP_C_REGS
-	addq	$8, %rsp	/* skip regs->orig_ax */
+	popq	%r11
+	popq	%r10
+	popq	%r9
+	popq	%r8
+	popq	%rax
+	popq	%rcx
+	popq	%rdx
+	popq	%rsi
+
+	/*
+	 * The stack is now user RDI, orig_ax, RIP, CS, EFLAGS, RSP, SS.
+	 * Save old stack pointer and switch to trampoline stack.
+	 */
+	movq	%rsp, %rdi
+	movq	PER_CPU_VAR(cpu_tss + TSS_sp0), %rsp
+
+	/* Copy the IRET frame to the trampoline stack. */
+	pushq	6*8(%rdi)	/* SS */
+	pushq	5*8(%rdi)	/* RSP */
+	pushq	4*8(%rdi)	/* EFLAGS */
+	pushq	3*8(%rdi)	/* CS */
+	pushq	2*8(%rdi)	/* RIP */
+
+	/* Push user RDI on the trampoline stack. */
+	pushq	(%rdi)
+
+	/*
+	 * We are on the trampoline stack.  All regs except RDI are live.
+	 * We can do future final exit work right here.
+	 */
+
+	/* Restore RDI. */
+	popq	%rdi
+	SWAPGS
 	INTERRUPT_RETURN
 
 
-- 
2.13.6

  parent reply index

Thread overview: 27+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-11-24  4:32 [PATCH v3 00/19] Entry stack switching Andy Lutomirski
2017-11-24  4:32 ` [PATCH v3 01/19] x86/asm/64: Allocate and enable the SYSENTER stack Andy Lutomirski
2017-11-24  4:32 ` [PATCH v3 02/19] x86/dumpstack: Add get_stack_info() support for " Andy Lutomirski
2017-11-24  4:32 ` [PATCH v3 03/19] x86/gdt: Put per-cpu GDT remaps in ascending order Andy Lutomirski
2017-11-24  4:32 ` [PATCH v3 04/19] x86/fixmap: Generalize the GDT fixmap mechanism Andy Lutomirski
2017-11-24  4:32 ` [PATCH v3 05/19] x86/kasan/64: Teach KASAN about the cpu_entry_area Andy Lutomirski
2017-11-24 13:16   ` Andrey Ryabinin
2017-11-24 16:27     ` Andy Lutomirski
2017-11-24 17:21       ` [PATCH v4] x86/mm/kasan: " Andrey Ryabinin
2017-11-24 19:26         ` Andy Lutomirski
2017-11-24 17:26       ` [PATCH v3 05/19] x86/kasan/64: " Andrey Ryabinin
2017-11-24  4:32 ` [PATCH v3 06/19] x86/asm: Fix assumptions that the HW TSS is at the beginning of cpu_tss Andy Lutomirski
2017-11-24  4:32 ` [PATCH v3 07/19] x86/dumpstack: Handle stack overflow on all stacks Andy Lutomirski
2017-11-24  4:32 ` [PATCH v3 08/19] x86/asm: Move SYSENTER_stack to the beginning of struct tss_struct Andy Lutomirski
2017-11-24  4:32 ` [PATCH v3 09/19] x86/asm: Remap the TSS into the cpu entry area Andy Lutomirski
2017-11-24  4:32 ` [PATCH v3 10/19] x86/asm/64: Separate cpu_current_top_of_stack from TSS.sp0 Andy Lutomirski
2017-11-24  4:32 ` [PATCH v3 11/19] x86/espfix/64: Stop assuming that pt_regs is on the entry stack Andy Lutomirski
2017-11-24  4:32 ` [PATCH v3 12/19] x86/asm/64: Use a percpu trampoline stack for IDT entries Andy Lutomirski
2017-11-24  4:32 ` Andy Lutomirski [this message]
2017-11-24  4:33 ` [PATCH v3 14/19] x86/entry/64: Create a percpu SYSCALL entry trampoline Andy Lutomirski
2017-11-24 11:39   ` David Laight
2017-12-01  5:56     ` Andy Lutomirski
2017-11-24  4:33 ` [PATCH v3 15/19] x86/irq: Remove an old outdated comment about context tracking races Andy Lutomirski
2017-11-24  4:33 ` [PATCH v3 16/19] x86/irq/64: In the stack overflow warning, print the offending IP Andy Lutomirski
2017-11-24  4:33 ` [PATCH v3 17/19] x86/entry/64: Move the IST stacks into cpu_entry_area Andy Lutomirski
2017-11-24  4:33 ` [PATCH v3 18/19] x86/entry/64: Remove the SYSENTER stack canary Andy Lutomirski
2017-11-24  4:33 ` [PATCH v3 19/19] x86/entry: Clean up SYSENTER_stack code Andy Lutomirski

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